cc: Remove LayerTreeImpl::root_layer usage outside tests

cc/trees/tree_synchronizer.h

 // Copyright 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CC_TREES_TREE_SYNCHRONIZER_H_
 #define CC_TREES_TREE_SYNCHRONIZER_H_
 
 #include <memory>
 
 #include "base/macros.h"
 #include "cc/base/cc_export.h"
 
 namespace cc {
 
 class LayerImpl;
 class LayerTreeHost;
 class LayerTreeImpl;
 class Layer;
 
 class CC_EXPORT TreeSynchronizer {
  public:
   // Accepts a Layer tree and returns a reference to a LayerImpl tree that
   // duplicates the structure of the Layer tree, reusing the LayerImpls in the
   // tree provided by old_layer_impl_root if possible.
   static void SynchronizeTrees(Layer* layer_root, LayerTreeImpl* tree_impl);
-  static void SynchronizeTrees(LayerImpl* layer_root, LayerTreeImpl* tree_impl);
+  static void SynchronizeTrees(LayerTreeImpl* pending_tree,
+                               LayerTreeImpl* active_tree);
 
   static void PushLayerProperties(LayerTreeImpl* pending_tree,
                                   LayerTreeImpl* active_tree);
   static void PushLayerProperties(LayerTreeHost* host_tree,
                                   LayerTreeImpl* impl_tree);
 
  private:
   TreeSynchronizer();  // Not instantiable.
 
   DISALLOW_COPY_AND_ASSIGN(TreeSynchronizer);
 };
 
 }  // namespace cc
 
 #endif  // CC_TREES_TREE_SYNCHRONIZER_H_